Use the correct path to impromptu-background.
Unreference the impromptu-background process.

Marking impromptu-background as `detached` is not enough, we also have
to inform the parent process to remove it from its references.
Try to read prompt as JS whenever possible

Compile CS to JS and cache it if your prompt uses CS

This saves about 50 ms
Conditionally load CoffeeScript

This saves about 50 ms
Extract helpers
Get rid of COMPILED_DIR variable

Makes prompts easier to test
Update tests to use their own compiled prompt location
Use buffer instead of specifying encoding

This seems to work best for 0.8